Your trail:

The Conservation and Restoration of Historical Stained and Painted Glass: An
Introduction to the Problems

The Conservation and Restoration of Historical Stained and Painted Glass: An
Introduction to the Problems

(Ivo Rauch)

1. Introduction

In all branches of learning in these enlightened
times there reigns yet still among us a certain barbarity in many areas.
Among other things, the unfortunate fate of most fired coloured glass panels
may serve as an example, and how many, indeed how very many of these
beautiful windows that were once the greatest glory of our churches have
been removed from our churches in our times, on the petty pretext of
allowing in more light. I say 'in our times', because the art of making this
glass has been lost, and we shall never be in a position to replace these
superb examples of workmanship.

The Frankfurt art connoisseur Heinrich Sebastian Hüsgen may have been only too
justified in penning these lines of complaint, in view of the heavy losses to
medieval window glass surviving in the late eighteenth century that could have
constituted just cause for such complaint.1
Yet without realizing it, this friend of Goethe had had the great good fortune
to see many of these precious old works of art in glass before the period of
major industrialization. During the course of the nineteenth and twentieth
centuries, losses to medieval churches and the glass windows with which they had
been furnished increased steadily on account of harmful environmental factors.
Concern has often been voiced that many historical examples of stained and
painted glass are today in an alarming condition, and are in need of cleaning
and conservation. A particular problem furthermore is the fact that many famous
panels have been restored on several occasions over the years, and often exhibit
patterns of damage that can be traced back to these restoration interventions.
Even stained and painted glass of the late nineteenth and early twentieth
centuries is now of an age that it will barely be possible to pass it to the
next generation without intensive maintenance and preventative conservation. In
view of the huge onus this constitutes in the preservation of historical
monuments, it is my intention here to explain some of the types of damage that
occur to historical windows, and to outline some of the measures with which
these types of damage can be addressed and prevented from happening in the
future.

As a result of their experiences, and following the catastrophic losses of the
two world wars, all European countries made particular efforts to preserve their
treasure troves of historical glass panels. Since all mid-European states
(primarily France of course, but also Germany and England) have large expanses
of medieval stained glass, methods for its cleaning and conservation were
developed everywhere. Different ways and methods of restoring historical glass
arose as a result of each country’s building on its native traditions.
Fortunately, over the last decade a common approach has developed, a body of
principles that remained unwritten for a long time, in accordance with which
restorers in many countries in Europe and overseas can deal with stained and
painted glass. This has arisen not least because of the efforts made by the
international Corpus Vitrearum Medii Aevi, whose committee for the restoration
of stained and painted glass drew up guidelines for the conservation and
restoration of stained and painted glass for the first time in 1989, in
conjunction with ICOMOS. These guidelines were recently reformulated by an
international working group and published in this form for the first time in
German (in Die Denkmalpflege, 62. Jg. (2004), Heft 2,
pp. 109-112), and in English (in S. Strobl and S. Trümpler, 'The
New International Guidelines for the Conservation of Stained Glass',
Journal of the British Society of Master
Glass-Painters, 28 (2004), pp. 168-74; and S. Strobl and S. Trümpler,
'New Guidelines for the Conservation of Stained
Glass', Conservation News, 94 (2005), pp.
34-36). This progress, welcome for addressing both the technical and the ethical
aspects of restoration, was occasioned by the establishment of academic training
courses for glass restorers in Antwerp, Paris and Erfurt, which are further
developing research and pedagogy in the field. Yet methodological and
technological differences remain still today between the different countries of
Europe. Consequently, what follows should provide an insight into the
restoration traditions of Germany against a background of principles that are
not the subject of international controversy; mention is also made of many of
the methods that differ from country to country.

2. The Production of Stained Glass

To begin, we should perhaps remind ourselves of the complex processes involved in
the production of stained-glass windows. Because the techniques used for
nineteenth-century and 'classic' stained and painted glass, which are still in
use even today, are those that were already being used in the Middle Ages, our
description of the process must begin with that period. The production of a
stained and painted glass panel in medieval workshops has been described in
detail frequently.2
Although in the most famous art-technological handbook of the Middle Ages, the
Diversarum Artium Schedula by Theophilus Presbyter
dating to the start of the twelfth century,3
the first step - the production of the glass itself - is described as if it were
part of the glass-painter's job, we should probably proceed on the assumption
that in reality glassworks were not in the same locations as the glass-painters'
studios, and that they employed different personnel. If nothing else, this was
necessary to satisfy the glassworks' enormous need for firewood.4
It is remarkable that the condition of medieval glasses today can vary greatly,
even within the same window. Even though the corrosion of glass is affected to a
variety of factors, we may conclude from this that as early as the thirteenth
and fourteenth centuries, workshops were simultaneously using differing base
materials from different, sometimes very distant glassworks.5

The starting point for the execution of the panel itself was a cartoon. This is
necessary from a purely technical point of view and is corroborated by
Theophilus. This sketch was very probably drawn onto a wooden panel covered with
a chalk whitewash, and an example of one such panel from mid-fourteenth-century
Gerona in Spain has survived.6
At the same time, sketches made on parchment were in use: these were more easily
transportable.7
Designs and cartoons on paper survive in increasing numbers from the late
fifteenth and early sixteenth centuries.8

The sketch constituted the basis on which the pieces of glass were cut. For each
colour desired, a piece of glass was cut out to the correct shape according to
its planned outline by means of a hot cutting-iron; the edges were then shaped
more precisely with a grozing iron. At later dates, diamond glass-cutters were
increasingly used for this purpose. The design for the interior surface was then
applied in a dark glass-paint and with washes of varying thicknesses; at later
dates silver stain9
and enamels,10
with which images could be modelled, were used to create the design. In
addition, the use of etching and cutting techniques, such as were applied on
occasion in the late middle ages and the nineteenth century, allowed the
layering of colours in multi-layered flashed glasses.11
After the painted glass had been fired in a wood-fired oven, the pieces of glass
were assembled by means of lead cames. These cames were cast to be H-shaped in
section, and in the medieval period they were shaved and cut with a knife to the
desired width and height. At later dates, the use of lead strips became the
norm, whereby the raw material could be milled cold into the desired
profile.12
When the leading was complete, the joints between the cames were soldered with a
lead and tin solder. It remains unclear whether the panels were then further
insulated against wind and rain with putty (as was normal in post-medieval
times), as there is no mention of putty in contemporary art-technological
handbooks, and demonstrating the presence of medieval putty on specific objects
is exceptionally difficult.13
The panels of glass assembled in this manner were usually installed directly
into the locations for which they were intended, in a groove in the stone
window-frame; in the Romanesque period, wooden framing systems were probably
used on occasion.14
Very often, however, there were ironwork framing systems, consisting of
wrought-iron flat bars, that subdivided the stone-framed openings in the window.
Panels were then secured in their stone frames with mortar and to the ironwork
with iron T-bars, against which the panels were placed and secured with wedges.
In later times, a whole variety of systems for holding glass panels in place
developed.15

3. The Deterioration of the Stained Glass

Some panels produced in this way have been able to last centuries in good
condition, as is demonstrated by the panels that have been kept in museums since
the beginning of the nineteenth century. However, those panels that have
remained in their original locations and been exposed to the weather and perhaps
suffered from inadequate maintenance today show signs of severe damage.
Particularly noticeable in the first instance are the thick layers of dirt that
cover the interior and exterior surfaces of nearly all historical panels. These
encrustations of dirt usually constitute a firmly attached stratum of dust, soot
(from candles and heating systems), remains from applications of putty, and
other noxious substances. The exterior surfaces can often be observed to have
been dirtied with pigeon droppings, which have an etching effect on the surface
of the glass. It is not just dirt that damages the panels: very often it can be
seen that the painting on the interior and exterior surfaces of the glass has
gone missing. The layers of dirt and the corroded paint layers are usually so
completely intermixed, that they can no longer be distinguished from each other.
These problems are not just found on finely painted figurative panels: they are
also found in ornamental glazing schemes, which today constitute the larger part
of those historical windows that survive.

An even more serious problem is the corrosion within the glass itself. This
starts as point-sized pits and spreads outwards from different centres (figs 1
and 2). The surface of the glass breaks up, and the pock-marked and damaged core
glass underneath becomes visible. This type of corrosion can cover the whole
surface of the glass and make it totally opaque. How can this type of damage be
explained in more detail? Images produced with a scanning electron microscope
show that the glass is divided up into strata (fig. 3). In glass of this type
and date, a zone builds up over the core glass where, over the course of time,
silicate molecules originating in the centre of the glass gather.16 The effect of this is to
render the zone harder and more chemically stable than the core glass.
Unfortunately, one characteristic of this so-called 'gel layer' is the fact that
it is riddled with tiny fissures and cracks. Water gets into the cracks and lead
to the formation of syngenite and gypsum, causing further damage. Both syngenite
and gypsum are opaque, which is why the glass seems to the viewer to become
gradually darker and eventually black; both are also precipitated in crystal
form. The crystals gain in volume, thereby breaking open the surface of the
glass. On top of this package of corrosion is the original, endangered paint
layer, as well as dirt and, very often, micro-organisms, such as mould or
algae.17

Another phenomenon causing serious damage is the browning of medieval glass at
the centre of the core glass, which can also lead to the glass's becoming
totally opaque (fig. 2). This phenomenon, usually caused by the oxidation of the
manganese present in the glass, is unfortunately quite common in glass of the
thirteenth and fourteenth centuries.18
In addition to the patterns of damage inscribed onto the glass by corrosion,
other forms of damage should be mentioned, such as the crazing of glass (as is
often seen in panels from the nineteenth century), as well as the many almost
banal-seeming cracks and gaps in historical windows, which are nevertheless very
expensive to deal with from a restoration point of view.

4. Restoration and Documentation

How should one proceed with historical stained and painted glass in light of this
complex damage situation? Before anything else, an absolutely essential
requirement for these valuable works of art is research by art historians and
restorers, which could include preparatory chemical analysis if need be. Even
taking this first step is usually beyond the capabilities of a traditional
glaziers' workshop, with the result that it becomes absolutely essential, right
at an early stage of such measures, to take on as project planners experts or
glass-restorers with specialist qualifications. While the various types of
browning that occur in medieval glass cannot be reversed in the present state of
restoration technology,19
in the case of the corrosion encrustations, appropriate interventions may be
made. It is normally useful to thin out the corrosion encrustations, as these
are hygroscopic, absorbing moisture, which can lead to further damage. It is
advisable initially to try to expose the various layers of corrosion:
increasingly intensive cleaning should be carried out, and samples investigated
with a scanning electron microscope, in order to establish the boundary between
the corrosion layer and the 'gel layer' (fig. 3). Ideally - though in practice
almost completely impracticable - the corrosion layer should be cleaned off
without damaging the underlying gel layer. This layer is chemically more stable
than the core glass, so less susceptible to corrosion. However, cleaning a piece
of glass too hard or too deeply will probably result in increased corrosion in
the future. In order to avoid this, restorers have gone over to leaving a thin
layer of corrosion material in situ. This uncontroversial, common-sense
approach has been adopted by all conscientious restorers and experts in Europe.
However, the ways in which people try to achieve the desired level of controlled
removal of corrosion encrustations are diverse. In view of the problems
associated with the glass surface for most stained and painted glass,
traditional cleaning methods are out of the question. Attempts to clean glass
with sandpaper, steel wool, steel brushes, sponges, dipping troughs, etching
acids or strong soapy water, which were still occurring in inexperienced
workshops some years ago, are thankfully a thing of the past. Instead of this,
different countries have developed different restorative cleaning methods. For
instance, in England work is often carried out with a micro-jet process, by
which the corrosion encrustations can be removed with exactly regulated
pressure.20
Comparatively soft materials are used for the micro-jet, such as crushed walnut
shells, sodium hydrogen carbonate, and wheat flour. In this way, the corrosion
layers can be thinned out in a focused manner. Many French restorers, in
agreement with the Laboratoires de recherche des Monuments historiques in
Champs-sur-Marne, prefer to remove corrosion chemically, by means of poultices
or gel pads.21
With this type of intervention, a cotton-wool poultice steeped in EDTA
(ethylenediaminetetraacetic acid, a loosening agent for chalky encrustations),
or a EDTA thickened to gel form, is placed directly onto the glass. After a
while, the poultice or pad can be removed and renewed if necessary. In the hands
of a specialist, this method can be used for deep, focused cleaning. However,
the suspicion remains that chemically cleaned surfaces corrode more swiftly, as
has been shown by experiments in Vienna, where sample areas of glass that had
been cleaned with EDTA developed new corrosion encrustations after only eight
years.22
In addition, the chemical method has the disadvantage with fragile painted glass
of endangering such broken remains of the paint as are found on the glass. In
such cases, work can be undertaken in a more controlled manner by using
mechanical means. Consequently, in Germany (and often in Belgium and the
Netherlands as well), there is a preference for working with mechanical means
(though sometimes with the help of chemical cleaning agents, such as ethanol
poultices) - with cotton buds and/or scalpels, with which the corrosion
encrustations are removed in layers.23 In most
cases, this type of cleaning takes place under the microscope. Many mechanical
means are available for further cleaning of the surface, such as different
varieties of hard brush, fat-absorbing powder, etc. Fortunately, opinion as to
these cleaning methods is no longer divided into camps. It is also agreed that
all three methods, in the hands of an inexperienced restorer, can cause
extensive damage. It is therefore very important only to employ specialized and
experienced restorers on a contract.24
Thankfully, in France, Belgium and Germany, glass restorers qualified to diploma
level are emerging in increasing numbers from the academic training institutions
mentioned above.

As already intimated, restoration work is particularly difficult when original
paint lines sit on top of the corrosion layers. These are often fragile and
prone to become detached (fig. 4). Cleaning should not take place as a first
step in such cases, and it may be necessary first of all to secure the original
paint outlines.25
This is done at specific points with small drops of synthetic resin, which are
applied under the microscope. Under no circumstances does it make any sense to
cover the whole piece of glass with synthetic resin. Such layers of resin are in
danger of becoming detached in a short space of time and tearing off the
painting underneath them, as has been shown by investigations on the windows of
the Church of St Martha in Nuremberg, which were completely covered in the
synthetic resin Paraloid B72 about twenty-five years ago. Here, the layer of
synthetic resin has developed large bubbles and is becoming detached from the
layer beneath over large areas. Layers of paint can also detach themselves from
the glass to which they are applied, even when there is no corrosion of the
glass. This often occurs in cases where enamel colours have been used. This
technique, developed in the fifteenth century, involved firing ground, coloured
glass onto support glass.26 Frequently, stresses
arise in the paint structure, on account of the differing expansion coefficients
of the support glass and the paint layer; the brittle support glass inevitably
suffers the effect of these stresses. The result is flakes that look like
crystals, which are very hard to secure in place and conserve.

A further significant problem is overpainting by earlier restorers. Oil- and
tar-based paints were used for overpainting on many windows, particularly in the
nineteenth and early twentieth centuries. Over the course of time, the
overpainting has become black, and is today working its way loose in the form of
large clumps, often taking large parts of the original painting with it.
Although there is often a desire to remove the overpainting on aesthetic
grounds, the decision requires careful consideration. In the first instance it
should always be clarified whether the 'cold', unfired painting layers are not
in fact retouchings by the artist; these should of course never be removed under
any circumstances.27
The dangers associated with the removal of overpainting mean that such serious
interventions should only be made when there are important conservation grounds
for doing so. For example, if the overpainting is endangering the historical
paint layers underneath, one should ascertain what the binding agent in the
overpainting paint is through chemical research. The layers of overpainting
should then be removed layer by layer with poultices, without damaging the
original layers of paint underneath by rubbing too hard or by over-wetting

There are often cracks and gaps in historical panels, and the way in which these
are treated from a restoration point of view throws up a large number of
questions. While the decision-making process for cracks in glass is relatively
simply, since repairing the crack with glue is only really necessary if there is
danger of further loss,28
the problems associated with missing areas of an image are harder to solve. A
fundamental principle that must be observed when restoring any object, is that
the work's authenticity should not in any way be compromised.29
Any works undertaken must respect what is preserved of the original state. In
the case of severely deteriorated stained and painted glass, there would be no
technical problem with creating an 'as new' version of what the glass may have
looked like - an extremely satisfactory solution for the lay public. However,
restoring severely deteriorated glass in this state would involve destroying
large parts of the object, and of course its historicity. The huge controversy
as to whether partly or totally destroyed works of arts should be reconstructed
has caused quite a stir in recent years.30
Unlike as in wall-painting (for example), the introduction of 'neutral'
stop-gaps of white or grey glass when restoring glass panels can lead to
excessive levels of brightness in these areas (on account of the sunlight
passing through them) and render the original composition much less legible.
Thus in each individual case a plan of action needs to be researched and
thoroughly reviewed. Results that are acceptable from an aesthetic point of view
as well as from a restoration-ethics standpoint can often be achieved with
suitably coloured and toned stop-gaps without any painted design on them.31

Because of the many different materials involved, restoration systems for stained
and pained glass are invariably highly complex. In addition to the different
types of glass and the glass surfaces, the leading and the putty sealing usually
also require restorative treatment.32 Often the joints between the leads have
broken and the leadwork is bowing out, which leads to multiple breakages in the
glass and compromises its water-tightness. Many lead cames suffer from corrosion
particularly around the solder points. It is often suggested that such areas
should be releaded, which is usually not necessary, and is also not acceptable.
Leadwork form part of the object's historicity, and are an integral component of
these works of art. For this reason, the leads should also be cleaned and
undergo restoration. Broken solder points can be resoldered, and if necessary,
the leadwork can be carefully reputtied on the unpainted side of the panel. Even
very damaged leadwork can be sufficiently restabilized and preserved in this
way. Mention should also be made of the stonework and ironwork that frame the
window. There are also important restoration decisions to be made here, on which
specialists should be consulted.33

It is self-evident that there should be written and visual documentation of all
measures adopted during restoration that may be handed down to future
generations. Standards for such restoration documentation have developed over
recent years.34
In all cases, consideration should be given to the suitability for archiving of
all materials relating to such documentation.35
Even in financially constrained projects, documentation is non-negotiable, as
later damage can often only be explained and correctly treated through knowledge
of a window's earlier history. Documentation is an integral part of the measures
taken and constitutes the concluding stage of every single restoration
project.

5. Protective Glazing

Many of the types of damage that occur to historical stained-glass windows can be
traced back to the effects of moisture over a long period of time, either in the
form of rain water, or condensation on the interior surface of the glass.
Moisture can cause and aggravate not just corrosion to the glass, but weathering
of paint layers and the corrosion of leadwork. In addition to their artistic
function, all windows were also intended to act as protection against the
weather and as space enclosers, and they were usually able to fulfil these
architectonic functions for centuries. However, if we wish today to preserve
these important works of art in the long term, the urgent necessity arises of
preventing the further effects of moisture on these sensitive surfaces. This
normally requires the installation of protective glazing, which prevents rain
and polluted outside air reaching the precious panels (fig. 5).36 If the decision is made to embark on a conservation measure of this
nature, a protective panel is installed in place of the original leaded glazing.
On the original ironwork support system is mounted a supporting structure that
extends into the interior of the building, on which the original leaded glazing
is placed, at a distance from the protective panel.37
It is absolutely essential to ventilate the space between the two layers of
glazing sufficiently, in order to minimize the occurrence of moisture in the
ventilation gap. It is recommended that the frequency of condensation on the
panels be checked with atmospheric measuring equipment in the interior of the
church and in the space between the original glass and the protective layer. If
protective glazing is installed, the various types of damage described above can
be greatly slowed, or even prevented. In addition, restorative intervention to
the historical panels can be limited to a conservatory minimum, as the painted
glass is relieved of its function of enclosing the space by the protective
glazing.

However, the installation of external protective glazing represents a major
intervention in the church's architectural structure, and must be weighed
against the desire to preserve historical stained and painted glass. One
particular problem that should not be glossed over is the fact that changes
arise from the repositioning of the historical leaded panels, both inside, where
the profiles of the stone framework are obscured, and outside, where the view of
the window is altered on account of the flatness of the protective glass panels.
While the change to the interior view is normally tolerated, huge efforts are
made in many European countries to mitigate the problem of the exterior view.
Many historical windows in Germany are fitted with protective glazing consisting
of hand-blown window glass (known as 'Goethe glass'). This glass contains small
irregularities, which enliven the exterior view of the building (cf. fig. 5).
Another way has been adopted in the new German states, where, in the context of
a project set up by the Deutsche Bundesstiftung Umwelt, many churches have been
fitted with protective glazing panels that have leadwork with rectilinear
interstices; this is intended to subdivide the large expanses of reflective
protective glazing. This solution does not seem very useful, because the
leadwork of the protective glazing cast shadows on the internal, original
glazing, thereby compromising its aesthetic impact.38
Many attempts have been made to use glass that has been given a matt finish.
From some viewpoints, these matt-rendered panels of glass are almost invisible,
but they have the significant disadvantage that from other standpoints they give
the effect of a completely opaque surface. A technically interesting solution
was developed by French restorers for Tours Cathedral.39 A mould was made of the external surface of the original leaded
glazing, and a clear glass panel was subsequently produced from this mould. This
glass panel reproduces all the irregularities of the medieval panel, including
the leadwork. At the next stage of the work, the external surface of the
protective panel was painted with various colours in imitation of the corrosion
encrustations of the original. As a result, the external appearance of the
protective glazing can hardly be distinguished from that of the original panels.
However, there is a problem with this system, in that the painted on colours
slightly reduce the amount of light in the interior, and distort such light as
does shine through. It would therefore be sensible not to proceed with any
painting of moulded panels. Yet the use of such heat-moulded panels poses the
basic question of the authenticity of the exterior view, because in the system
described, large areas of architecture are filled with 'imitation' leaded
glazing.40
One very simple and often very useful method of improving the visual impression
of protective glazing is the installation of woven-wire grilles, such as have
been used for centuries as protection against stones thrown at windows. The
grating disguises the flat protective glazing behind it and can be a good
solution, particularly for buildings where it can be proved that historically
such gratings were fitted. If windows that require work are masked by nearby
buildings, it is perfectly possible to dispense with expensive, specially
produced glass, and to use cheaper window glass or Goethe glass to make up the
protective glazing; this has the advantage of allowing the maximum possible
transmission of light. As can be deduced from the variety of possible solutions,
it is always absolutely essential to test the effect of the different systems on
the building structure itself with different models, before embarking on the
reconstruction of whole groups of windows. Solutions that may have provided very
satisfactory results in one case cannot be transferred wholesale to another
building.

6. Final Observations

It remains only to emphasize that in the restoration of historical stained and
painted glass there are no 'patent solutions' that could have been detailed in
the course of the present article. Both the complex nature of the damage to
historical windows and the aesthetic problems of protective glazing demand
intensive preparation and research in each individual case. A plan for the
restoration of a particular object can only be drawn up on the basis of
technical, art-historical and scientific research. Only then is it possible to
put out to tender a detailed, object-specific list of the works to be undertaken
by qualified glass restorers.41
In view of the difficult questions posed by works of art to those working on
them, it is imperative that specialists from different areas work together while
the restoration is being carried out. At least three partners should be closely
involved: the client; the expert and/or architect contracted by the client; and
the restoration workshop carrying out the work. Responsibility also falls on the
appropriate building authorities and monument curators. With objects of such
importance it is also sensible to involve an expert from the relevant national
committee of the Corpus Vitrearum Medii Aevi. In addition to expertise in the
field of glass restoration, an important prerequisite is therefore collegiate
co-operation in working groups, so that our precious stained-glass heritage may
be conserved and preserved for the future.